The Gejiu alkaline complex is composed of alkaline feldspar syenite in border facies and nepheline syenite in central facies. The whole-rock geochemical analysis shows that the alkaline complex has the characteristics of high alkalinity, rich potassium, rich iron, low magnesium, and highly differentiated alkaline-peralkaline rocks, with the late stage more enriched in alkali metal elements. LREE/HREE=20~59, (La/Sm)N=8~50, and (Sm/Yb)N=1.2~5.0. It is rich in light rare earth elements. The fractionation degree of the light rare earth elements is higher than that of the heavier rare earth elements. It has Eu negative anomaly. The Gejiu alkaline complex is depleted in such elements as Ti, Nb, P, K, and Sr, and rich in such elements as Zr, Hf, Th, La, Ce, Nd, U and Rb. All these characteristics indicate that the magma source is related to mantle derived materials. Alkali syenite and nepheline syenite have similar characteristics of trace elements and rare earth elements, and have the characteristics of homologous magmatic differentiation and evolution. The ratios of Rb/Sr, Nb/Ta, and Zr/Hf are all higher than or close to the corresponding values of the original mantle. CIPW standard mineral calculations show that perilla pyroxene, cone pyroxene, and olivine appear in the alkali syenite in marginal facies. Olivine appears in the nepheline syenite in central facies. Combined with the characteristics of the ratios of (Th/Nb)N and Nb/La and the research results of previous Sr-Nd isotope, it is considered that the magma of Gejiu alkaline complex was derived from partial melting of enriched mantle subjected to metasomatism and limited crustal contamination, which was formed in the extensional environment of the post-collision. In the late period of alkaline magama evolution, alkali was richer, and the higher degree of crystallization differentiation constituted an important factor for the abnormal enrichment of REE, Nb, Ga and Zr.